Updated the digital document signing system to use INCHES as the standard unit for annotations and signatures, aligning with GdPicture14's native format. Previously used DevExpress units (1/100 inch) and other formats have been replaced.
- Updated `AnnotationDto` to reflect the new coordinate system.
- Introduced `SignatureDto` for signature positions, deprecating `AnnotationDto`.
- Added conversion formulas for transforming coordinates between INCHES and other systems (DevExpress, PDF Points, PDF.js, etc.).
- Added a unit comparison table and A4 page dimensions in various units.
- Introduced a new read-only PDF.js viewer for envelopes (`/envelope/{EnvelopeKey}`).
These changes improve consistency, simplify conversions, and align with modern tools like PSPDFKit and iText7.
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